Preheat the oven to 400°F (200°C).
Heat 1 tablespoon olive oil in a large skillet over medium heat.
Add the cubed chicken and cook, stirring occasionally, until browned and cooked through, about 5 minutes; transfer to a plate with a slotted spoon.
Bring a large pot with 4 cups (or enough) of water to a boil. Add the jumbo shells and cook until al dente, about 10 minutes. Drain thoroughly in a colander and let cool slightly so shells do not tear.
In a mixing bowl, combine the cooked chicken, 15 ounces ricotta, three-quarters of the shredded Parmesan (reserve remaining for topping), and 1 tablespoon Italian seasoning. Stir until evenly mixed.
Spoon a thin layer of marinara sauce into the bottom of a 9×13 baking pan.
Fill each cooked shell with the chicken-ricotta mixture and place filled shells in the prepared baking dish in a single layer.
Pour the remaining marinara sauce evenly over the filled shells and sprinkle the remaining Parmesan cheese on top.
Bake in the preheated oven for 20 minutes until heated through and cheese is melted. Remove and serve.
